This invention relates to apparatus for the separation of solid matter from a liquid by magnetic attraction or by both magnetic attraction and filtration. The solid-liquid separation apparatus in accordance with the invention has particular utility in conjunction with the clarification of printing ink; nonetheless, it lends itself to the processing of petroleums, organic solvents, paints, dyes, etc., and to water purification as well.
Let us consider gravure printing as an illustration of the need for the removal of solid particles from printing ink. In gravure printing the image area is etched, engraved or otherwise created below the surface of the printing plate or cylinder in the form of tiny "cells" or depressions. The printing surface is flooded with ink and then wiped or scraped clean of excess ink by a steel blade, called a doctor, leaving the ink contained in the cells below the surface. The ink is transferred from the cells to the paper pressed against the surface.
If the gravure printing ink contains solid matter, it will become caught between the printing surface and the doctor and so interfere with the thorough scraping of excess ink off the printing surface. The solids will also cause the rapid or uneven wear or damage of both the printing surface and the doctor, materially shortening their service life. The worn or damaged printing surface and doctor can impair the quality of the reproductions and even produce defective prints that must be discarded. These results greatly increase the manual labor of removing the foreign matter from between the printing surface and the doctor, reconditioning or replacing the printing surface and doctor, and inspecting the produced prints for defective ones. The consequent increase in printing cost is also a problem.
Further the solid matter contained in printing ink may seriously affect the equipment through which the ink flows or recirculates. An example of such equipment is an automatic viscosity control provided for the ink reservoir forming a part of the closed path for ink recirculation. The viscosity control has a rotor for sensing ink viscosity from the resistance offered by the ink to its rotation. The rotor is received with clearance in a tubular sheath, and the ink flows through the narrow clearance space therebetween. Should any coarse solid be caught between the rotor and its sheath, the viscosity control would react as if the ink viscosity had increased.
A complete removal of solid particles from gravure printing ink is the only possible solution to the foregoing problems. A variety of solid-liquid filters or separators have been suggested and used in the printing industry. Typical of these are meshed or screen filters, roll-up filters, cartridge filters, and a combination of a screen filter and magnetic means. All these prior art devices have one drawback or another.
Screen filters rapidly deteriorate in their filtration ability with the clogging of the pores. They must therefore be backwashed periodically to clean them of accumulated solids, with the filtering operation suspended. Roll-up filters are susceptible to rupture due to the liquid pressure. Being not reusable, moreover, they are expensive and also require a break in the filtering operation for the replacement of the used rolls.
Known apparatus comprising both a screen filter and magnetic means for solid-liquid separation is also subject to several objections. One of these concerns the arrangement of the magnetic means with respect to the flow path of the liquid. As all the liquid does not pass sufficiently close to the magnetic means, the magnetic matter contained in the liquid is not removed therefrom to a desired degree. Additionally the screen filter for combined use with the magnetic means has its own disadvantages set forth. previously.
Cartridge filters are commercially available with facilities for cleaning the cartridge without interruption of filtering. The cartridge consists of a stack of flat wheel shaped discs, arranged alternately with spacers to provide spaces between the discs for filtration. Fixed alongside the stack of discs is a comb or a set of stationary cleaner blades which extend into the spaces between the discs. When the cartridge is turned through a complete revolution, the solids are combed out of the spaces between the discs by the cleaner blades.
Although the cartridge filters can be reconditioned as above without interruption of filtration, they are limited in the size of particles that can be filtered off, and so are unable to capture fine iron or other metal particles which do much harm in gravure printing. Another disadvantage is that the cartridge is easy to clog up unless it is cleaned at intervals determined in accordance with the actual flow rate of the liquid therethrough, rather than periodically, particularly in cases where the liquid contains a large proportion of solids.
A further drawback of the conventional cartridge filter arises from the fact that the stack of discs is turned in one and the same direction for scraping off the solids by the cleaner blades. The unidirectional rotation of the discs causes the solids to attach only to and accumulate on one side of each cleaner blade, resulting in rapid deterioration of the cleaning ability of the cleaner blades. The use of a high speed electric motor for the revolution of the disc stack in accordance with the prior art is also objectionable as it necessitates the combined use of a speed reducer; the complete motor drive unit has been too bulky and expensive.
A still further objection to the conventional cartridge filter is that the cleaner blades are mounted in fixed relation to the stack of discs, usually with some clearances between the cleaner blades and the peripheries of the spacers which are also in the form of apertured discs smaller in diameter than the filter discs. In the presence of such clearances the cleaner blades can remove coarse or rigid solids but not paper fibers which abound in printing ink. Sticking fast to the discs, the paper fibers cannot possibly be combed off unless the cleaner blades are pressed firmly against the discs.
Scraped off the cartridge, the solids settle to the bottom of the housing accommodating the cartridge. They can normally be discharged by opening a drain port at the bottom of the housing. However, if the amount of solids is unusually large, as after a prolonged period of filtering operation, the drain port has been easy to clog as the accumulated solids stick together, making it impossible to discharge them. The prior art cartridge filter has had no provisions for preventing such clogging of the drain port.